Field of the Invention
The control of noise and vibration in composite structures is an important area of current research in aerospace, automotive and other industries. For example, spacecraft vibrations initiated by attitude adjusting thrusters, motors and thermally induced stresses inhibit accurate aiming of antennas and other equipment carried by the craft. Such vibrations can cause severe damage to the craft and its associated equipment. Fatigue failure of structural components can occur at stresses well below static load limits.
Traditional passive noise and vibration control methods are heavy, bulky, and perform only marginally. For example, the acoustics of aerospace vehicles during launch are severe enough to cause damage to payloads and guidance control systems and could cause failure of the mission. Typically, standard metallic and composite technologies rely upon the use of heavy acoustic blankets to reduce the damaging effects of sound pressure fields during launch. Structurally amplified acoustic and vibration energy exacerbates the problem due to low inherent damping in fairings and other structural components. A practical way of increasing damping and improving acoustic properties in mechanical structures is required.
Composite materials have been used to construct a wide variety of structural elements, including tubes, enclosures, beams, plates and irregular shapes. Objects as diverse as rocket motor housings and sporting goods, notably skis, archery arrows, vaulting poles and tennis rackets have been structured from composite materials. While composite constructions have offered many significant advantages, such as excellent strength and stiffness properties, together with light weight, the poor vibration damping properties of such construction have been of concern.
The invention relates to fiber reinforced composite structures and applications that use wavy fiber patterns in the plane of the laminate, and that increase damping with little or no sacrifice in strength.
The invention also relates to the methods and apparatus for manufacturing the aforementioned composite material structures.
Another aspect of the invention is directed toward the fabrication of a wavy fiber pre-preg (fibers preimpregnated with epoxy resin). Such pre-pregs not only have an aesthetic appeal but also may be fabricated with selected variable volume fractions to accommodate a variety of applications.
Description of Related Art
The following terms used herein will be understood to have their ordinary dictionary meaning as follows:
Fiber: a thread or a structure or object resembling a thread. A slender and greatly elongated natural or synthetic filament. (Includes metal fibers) PA0 Matrix: material in which something is enclosed or embedded. PA0 Viscoelastic: having appreciable and conjoint viscous and elastic properties. Note: a special case of the term "viscoelastic" is "Anisotropic Viscoelastic", defined below. When the term viscoelastic is used in the following text it should be construed to encompass this special case. PA0 Lamina(te): a thin plate . . . LAYER(S) PA0 Composite: made up of distinct parts. PA0 CWC: (Continuous Wave Composite) defines any fiber-matrix combination having at least one fiber without a break (or interruption) and having a pattern which can be defined by a mathematical algorithm. It generally has a wavy appearance. It can consist of "unidirectional" fibers (although in this case the fibers would be placed in a wavy pattern) or woven cloth (which also will have a wavy pattern to the warp or weft). PA0 CWCV: (Continuous Wave Composite Viscoelastic) defines a combination of CWC and viscoelastic materials designed to induce damping in a structure. PA0 CWC-AV: (Anisotropic Viscoelastic) defines a viscoelastic material or matrix with an embedded wavy fiber pattern. Such a material would have anisotropic, elastic, and viscoelastic properties. It is a special case of both CWC laminates and "viscoelastic" and can be used in conjunction with conventional CWC fiber-matrix combinations to provide damping and unique structural properties. Any use of "CWC" or "viscoelastic" in the following text can be construed to encompass this special case. PA0 1. Pratt, W. F., Rotz, C. A. and Jensen, C. G. 1996 "Improved Damping and Stiffness in Composite Structures Using Geometric Fiber Wave Patterns," Proceedings of the ASME Noise Control and Acoustics Division, Vol. NCA 23-2, pp. 37-43. PA0 2. Pratt, W. F., Rotz, C. A., and Jensen, C. G., 1996, "On the Use of Continuous Wave Composite, Structures in Stress Coupled Interlaminar Damping," Advanced Materials: Development, Characterization Processing, and Mechanical Behavior Book of Abstracts, Vol. MD 74, pp. 63-64. PA0 3. Pratt, W. F., Rotz, C. A., and Jensen, C. G., 1996, "On the Use of Continuous Wave-like Geometric Fiber Patterns in Composite Structures to Improve Structural Damping," Proceedings of the ASME Aerospace Division, Vol. AD 52, pp. 415-433.
One of the simplest and often very effective passive damping treatments involves the use of thermo-viscoelastic (TVE) materials. These materials, represented by Avery-Dennison's FT series (FT-1191 is one example), exhibit both elastic and dissipative qualities which make them useful in a number of passive damping treatments.
Some of the first uses of thermo-viscoelastic materials to increase structural damping involved the use of surface patches of aluminum foil and viscoelastic adhesives. Called constrained or embedded-layer damping, these methods produce modest gains in damping.
One of the more common passive damping methods, Constrained Layer Damping or CLD (Kerwin, 1959), is achieved by bonding a thin layer of metal sheet, usually aluminum, to an existing structure with a viscoelastic adhesive. Shear strains develop in the viscoelastic material when the original structure bends or extends. Damping occurs when the deformation of the viscoelastic adhesive creates internal friction in the viscoelastic material, generating heat and thus dissipating energy.
Compared to an undamped structure, this approach, is modestly successful but its effectiveness decreases markedly as the ratio of the thickness of the base structure to the thickness of the viscoelastic material increases (Hwang and Gibson, 1992). Thus surface treatments alone cannot provide significant levels of damping to structural members where greater strength and stiffness are important. Hwang and Gibson (1992) reported this problem and showed that the advantage of aluminum foil viscoelastic constrained layer damping was eclipsed by the inherent damping in conventional composites when the required thickness of the structure exceeds about eight millimeters .3 inches). They determined that a .+-.45.degree. graphite/epoxy composite provided approximately uniform damping of about 1.5% in thick sections, that was at least one order of magnitude greater than comparable aluminum structures.
Co-cured composite-viscoelastic structures are formed when layers of uncured fiber composites and TVE (thermal-viscoelastic or viscoelastic) materials are alternately stacked and cured together in an oven. Damping occurs in these structures when a load causes differential movement of the opposing laminates, causing shearing in the sandwiched viscoelastic material. The various methods that use this concept of differential shearing of the viscoelastic material can be classified by the fiber orientation methods used to induce damping in the TVE material.
Conventional angled ply composite designs use .+-..theta. lay-ups of straight fiber pre-preg materials to encase the viscoelastic layers, and were first proposed by Barrett (1989) in a design for damped composite tubular components. Barrett combined the concepts of constrained layer damping with anisotropic shear coupling in the constraining composite layers to create a tube that achieved both high damping and high axial stiffness. Barrett's research showed that maximum shearing was experienced at the ends of the tubes and that clamping the constraining layers of the tube at the ends eliminated much of the damping effect, rendering the design impractical for most applications.
Chevron patterned designs also use conventional angled ply (.+-..theta.) composite lay-ups of straight fibers but vary the fiber orientation several times throughout the structure in a given laminate. Called SCAD (Stress Coupled Activated Damping), it was first proposed by Benjamin Dolgin of NASA and implemented by Olcott et al. (1991a).
In Olcott's implementation of Dolgin's design, each composite layer is comprised of multiple plies of pre-preg composite material arranged in a series of chevron-like patterns. Each composite layer is also comprised of several "segments" of material where the fiber angle in a given segment is oriented in a single direction throughout its thickness. Segments on opposite sides of the embedded viscoelastic material have the opposite angular orientation. At least two adjacent segments in a given composite layer are required to form a chevron and are joined together by staggering and overlapping the pre-preg plies in the segment.
By tailoring the fiber angle, thickness, and segment lengths, significant shearing in the viscoelastic layer was observed over the entire structure, not just at the ends as in Barrett's design (Olcott et al., 1991b; Olcott, 1992).
Olcott's research showed that the fiber orientation, segment length, segment overlap length, material choice, and material thickness, had to be carefully controlled to maximize damping in a structure (Olcott, 1992). His best design, built and tested, was a 51 cm (20 inch) tube that used a fiber lay of .+-.25.degree. and a segment length of 3.8 cm (1.5 inches). This single damping layer tube produced almost 9% damping in the axial mode. Olcott also experimented with the use of chevron damping patterns in the flanges of a composite "I" beam with good success [Olcott, 1992 #19].
Pratt, et. al. [Pratt, 1997 #105] proposed several processes for making the wavy composites contemplated by Dolgin, their use in combination with viscoelastic materials for increased damping in composite structures, and the manufacture and use of several specialized wave forms.